Dominic Johnson-Hill explores the tunnels under Beijing for possibly the last time.Meters beneath Beijing’s hectic streets, a warren of bomb shelters extends citywide, dug at the height of nuclear paranoia in the 1970s.

Home to sewer rats, damp and more recently, migrant workers, they’re fast being filled in and forgotten. Seizing what might be the last chance to see the tunnels, a friend and I don flip-flops and a flashlight for a final descent.

“You're in luck -- he's drunk”. Dominic Johnson-Hill reaches over the sprawled, snoring body of the building warden and snatches a flashlight from the cluttered desk.

We’re in Nanluogu Xiang, a gentrified neighborhood of grey-brick alleyways north of the Forbidden City.

Johnson-Hill, owner of nearby design store Plastered T-Shirts and self-styled steward of this tunnel entrance, has been inside 30 or 40 times. “It’s like a secret world, with so much to discover, it became an addiction,” he confesses.

“We’re beneath a government building, so the tunnels go deeper, and the construction is better," says Johnson-Hill. "These would have belonged to a senior official.”

Some way along a side tunnel, two bare, concrete rooms face each other: sleeping quarters and a smaller office housing a single desk thick with mold.

Above, an escape hatch is tunneled into the ceiling, a rusty iron ladder poking down. The air is chilly and damp.The tunnels were originally intended to be an escape from a Russian attack.

Underground escape

Though reliable details are scarce, Beijing’s “underground city” was claimed to cover around 80 square kilometers, dug by many thousands of conscripted civilians at the height of tensions with Soviet Russia.

The plan was that most of central Beijing’s inhabitants could scurry underground within minutes of an attack, and stay there safely for weeks if necessary.

Wang, a Nanluogu Xiang resident, remembers it well. “I was about 15 when the tunnels were built. They were dug in a hurry and entrances started popping up around our neighborhood. Most of us weren't allowed in, and we had a lot on our minds in those days so people soon forgot about them."

But as recently as 2007, the tunnels were a tourist attraction of sorts. Visitors could take in a broad loop of musty tunnels southeast of Tiananmen Square.

Famously hard to find, I sought them out in early 2008 only to discover a note pinned on a door: “Closed until further notice.”

It still is. Greg Savarese, a coffee entrepreneur from the United States, managed to see the tunnels on a visit in 2004.

“We searched for ages and eventually found some soldiers who unlocked a door and took us below ground -- we walked for about an hour and saw a school, a hospital, street signs on the walls -- even a movie theater,” he says.

Splashing through ankle-deep puddles, Johnson-Hill points out a faded Mao inscription on the wall: "Dig deep, store grain, don't claim to be an emperor."

Nearby, a dog-eared map hints at the scale of the surrounding network, revealing dozens of rooms, corridors, storage areas and access tunnels.

But we can’t go any further. The tunnel is bricked up.

“The first time I was down here it was a much bigger place -- over 600 meters of tunnels, some wide enough to take a car,” reports Johnson-Hill.

So what happened?

“Either they were destroyed by the subway network or sealed for security reasons," he says. "I remember you could literally emerge from the tunnels and pop your head into someone’s beautiful old courtyard home.”Now some areas under Beijing could be developed into shopping malls and roads to ease life above ground.

Safety first

David Eimer, author of Lonely Planet Beijing, told me the reason the tunnels are being trashed is because of urban redevelopment. “Newer, bigger buildings require deeper foundations, so many of the tunnels have to be filled in for safety reasons.”

Back on the surface, a pile of bricks has been deposited outside the building entrance. Two workers begin carrying them down into the tunnel we’ve just emerged from.

A day later Johnson-Hill reports that two more side tunnels have been sealed up.

It’s the same story a short distance away in Fangjia Hutong.

For several months last year I’d been taking friends down a staircase beside a restaurant on the site of an old factory. It led to a warren of low-ceilinged, pitch-black tunnels, descending deeper and deeper until completely filled with water.

Now it too is closed.

Eventually, all traces of this paranoid precinct will be buried, and not just because of new subway lines or high-rise development.

The space beneath Beijing is increasingly seen as a means of easing the strain on a city overloaded with people and traffic.

The state-run China Daily reported in 2010 that city planners have earmarked three underground levels to be built beneath Dongcheng district, housing shopping malls and roads.

Should this happen, I wonder if future subterranean Beijingers, browsing rails of designer clothing, will feel any echo of the chilling "underground city" that once existed in its place.

A food and travel specialist, Tom has munched his way through the capital’s best kaoyadian in search of the perfect Peking duck, journeyed along the former Silk Road to the distant sands of Kashgar, grappled a baby panda in Sichuan, and generally counted himself lucky for being witness to an era-defining period of Chinese history. He has written for The Guardian, Travel & Leisure, Fodor’s, Time Out and the South China Morning Post, and blogs at www.tomfreelance.com.